The present invention generally relates to the field of Raman spectroscopy, and more particularly, to a sensor for detecting chemicals both in gas and liquid environments using surface enhanced Raman spectroscopy.
Raman spectroscopy is an emission technique that involves inelastic scattering of incident laser energy and results in spectral peaks that are frequency shifted from the incident energy. The Raman bands arise from changes in polarizability in a molecule during vibration. As a result, virtually all organic molecules display a characteristic Raman emission. Therefore, a Raman sensor would not be limited to a specific class of molecules as is the case for the laser induced fluorescence (LIF) sensor. Raman spectrometry allows the fingerprinting of species present and is structurally specific. The inherently high resolution of Raman spectra often permits the analysis of several components in a mixture simultaneously.
The advent of inexpensive, portable Raman spectrometers has seen renewed interest in the area of Raman spectrometry. This new generation of spectrometers employs fiber-optic probes, holographic notch filters for rejection of the Rayleigh line, a single grating monochromator, and a charge-coupled device (CCD) detector for multichannel detection. These spectrometers contain a minimum of optical components as compared to conventional Raman instrumentation resulting in high throughputs; and, once coupled to a laser and spectrometer, optical-fiber probes require no further alignment.
Despite the advantages of Raman spectroscopy over other spectroscopic techniques and the technological advances in the area of Raman spectrometry, Raman spectroscopy is, inherently, an insensitive technique. To achieve detection limits in the low ppm range would require either the use of a multiple pass cell or long acquisition times. In the 1970s, it was discovered that Raman scattering from molecules adsorbed on such noble metals as silver, copper, and gold can be enhanced by as much as 106 to 107. This phenomenon, called surface enhanced Raman spectroscopy (SERS), is still not understood despite intensive theoretical and experimental research. It is believed that more than one mechanism is involved in the SERS phenomenon. Initially, the SERS technique was used as a means to probe adsorption at metal interfaces both in electrochemical and gas-phase environments. This technique has proven useful in deducing the effects of interfacial structure and reactivity on the adsorption process. However, the sensitivity of the technique as well as its exceptional spectral selectivity has made SERS attractive for a broad range of analytical applications. SERS can be used for trace organic analysis and as a detection method in gas chromatography, liquid chromatography, and thin layer chromatography. Electrochemical SERS and SERS of chemically modified surfaces have been used to detect aromatic compounds and chlorinated hydrocarbons, organic contaminants of environmental concern, in the ppm concentration range.
There are many applications in which detection of particular chemical species or analytes is desirable, as for example, hydrocarbons that may be present in ground water, toxic vapors in industrial environments, explosives, metal ions, narcotics, toxic anions, and chemical warfare agents.
However, a problem with optical fiber based SERS systems is that the optical excitation signal, and Raman emissions received by the collection optics can generate secondary Raman emissions in the optical fibers. Therefore, a need exists for an optical fiber based SERS sensor for detecting analytes of interest which is not affected by secondary Raman emissions within the excitation and collection fibers. A further need exists for an optical fiber based SERS sensor that may be deployed in physically challenging environments, such as at sea and in terrestrial bore holes.
The present invention provides a sensor for performing surface enhanced Raman spectroscopy (SERS) that includes a sensor body having a throughbore; a window mounted to the sensor body that is coterminous with the throughbore; a surface enhanced Raman scattering structure mounted to the window; an optical energy source for generating an optical excitation signal; a first optical fiber mounted in the throughbore for directing the optical excitation signal through the surface enhanced Raman scattering (SERS) structure; a second optical fiber mounted in the throughbore for receiving primary Raman emissions generated when an analyte in contact with the surface enhanced Raman scattering structure is irradiated by the optical excitation signal; and an optical detector for generating an optical signal representing the primary Raman emissions. A long pass filter is optically spliced in series with each second optical fiber for filtering out optical signals having wavelengths that are less than a predetermined wavelength. The sensor also includes a bandpass filter optically spliced to the first optical fiber for attenuating any secondary Raman emissions that may be stimulated in the first optical fiber by the optical excitation signal. A first lens collimates the excitation signal from the first optical fiber and another lens focuses the excitation signal onto the external surface of the SERS structure, i.e., at a SERS surface/liquid interface or SERS surface/gas interface when the sensor is being utilized. The sensor may further include electrodes for polarizing the SERS surface to attract analytes to the surface. The sensor may further include a liquid detector for determining when the sensor body is in contact with a liquid. Another embodiment of the invention includes electrodes in the vicinity of the SERS structure for performing electrochemical SERS.
The surface enhanced Raman scattering structure includes: a glass substrate having a roughened surface; an adhesion layer formed on the roughened surface; metal islands formed on the adhesion layer that define a metal patterned structure; and a self-assembled monolayer formed over the metal islands.
The sensor eliminates background interferences arising from fiber emissions, operates at long lengths of fiber (30+ meters), is able to do multiple samplings, is easily deployable, and withstands the shock and vibration associated with deployment in subsurface environments. The sensor may be used to detect subsurface pollutants of environmental concern, in particular BTEX, chlorinated hydrocarbons, anionic nutrients, metal ions, narcotics, explosive materials, and agents of chemical warfare.